Broadband source process of the 1998 Iwate prefecture, Japan, earthquake as revealed from inversion analyses of seismic waveforms and envelopes

An earthquake of M 6.1 occurred on 3 September 1998, along an active fault at the southwestern foot of Mt. Iwate, a volcano in northeastern Japan. Acceleration records of this earthquake were obtained at seven stations within 40 km of the epicenter. In order to investigate the source process of this earthquake in a broad frequency range, we simultaneously conducted inversion analyses of low-frequency seismic waveforms and high-frequency seismogram envelopes. First, executing the envelope inversion by using the envelope Green function derived from the radiative- transfer theory in the high-frequency band of 2-16 Hz, we estimated the spatial distribution of seismic-wave energy radiation on the fault plane of 10 km x 10 km. We found that seismic-wave energy was strongly radiated from the southwestern deeper part of the fault plane. By using data from the same stations and using the same fault geometry as the high-frequency analysis, we applied a waveform-inversion method to three-component displacement records in the low-frequency band of 0.1-0.33 Hz. This result showed that seismic moment was mainly released at the shallow part of the fault. Comparing these results, we found that high-frequency energy was strongly radiated from the deepest periphery of the region, where seismic moment was mainly released. This result implies that the radiation of high-frequency energy was associated with the arrest of rupture for this earthquake.